Space operations involving space stations and long duration space flights have created an awareness of the critical need to protect spacecraft structures from the hazard presented by orbital debris. While shielding schemes devised for protecting against micrometeroids have met with success, the problem of defending against the more dense and slower moving particles of orbital debris has not heretofore been satisfactorily solved. These particles have densities of about 2.7 g/cm.sup.3 and impact velocities up to 16 km/s as compared with densities of about 1 g/cm.sup.3 and average impact velocities of 20 km/s for micrometeoroids. Previous schemes for protecting against the typically slower particles of orbital debris have included single sheet shields and dual or multi-layered shields for protecting a wall. The sheets in these prior art schemes are often characterized as "thin" with a ratio of sheet thickness (t.sub.s) to the theoretical diameter (d.sub.p) of a spherical impacting particle which is in the range of 0.15 to 0.25. With such shields, each sheet adds material to the debris plume which can damage the back wall. They also produce secondary ejecta into the space environment where they pose additional hazards to space vehicles or other space structures in the line-of-sight of their trajectory.
For a conventional hypervelocity shield concept or design which consists of a single or multi "thin" sheet shield, the orbital debris upon impact typically fragments into a large number of fine solid debris projectiles that are hot but not molten. In their contact with subsequent "thin" sheets of the shield, more mass is added to the debris plume by the impact process with the result that each "thin" sheet does not assist the process of destruction as much as it adds more destructive material to impact the next sheet. In most instances, each of these "thin" sheets acts as a "choke" to constrain the debris or cloud plume from expanding. The net effect is the need for a very thick spacecraft wall to defeat the debris energy.
A paper entitled, "Development of Dual Bumper Wall Construction for Advanced Spacecraft" by A. J. Richardson and J. P. Sanders appearing in the JOURNAL OF SPACECRAFT AND ROCKETS, Vol. No. 6, June 1972 discloses a multi-sheet shield and recognizes that a second sheet can fragment the fragments created by the first sheet, thereby allowing for a reduction in wall thickness. The first sheet is characterized by a ratio of sheet thickness to particle diameter equal to 0.13 and the failure mode of the wall is changed to "bulge and tear" rather than perforation.
In the patented prior art, U.S. Pat. No. 3,439,885 discloses a protective shield for spacecraft which comprises a thin outer wall and layer of bronze wool secured to the outer surface of a load-carrying structural wall of the spacecraft.
U.S. Pat. No. 3,575,786 discloses a shield interlayer for spall suppression which comprises a nylon felt layer adjacent the inner surface of the shield and a urethane elastomer of high elasticity and tear resistance which is bonded over the surface of the felt layer.
U.S. Pat. No. 3,771,418 discloses an anti-spall light-weight armor which includes a shock absorbent layered combination of fiber glass materials with a resinous bonding material.
U.S. Pat. No. 4,198,454 discloses a lightweight composite shield for resisting penetration by small arms projectiles which comprises a first panel of a multi-layered construction of metal panels spaced by a honeycomb structure filled with a subliming material, a further panel of projectile resisting material, and thermal insulation material disposed therebetween.
U.S. Pat. No. 4,664,967 discloses a ballistic spall liner in the form of a laminate of layers of high tensile strength woven fabric bonded together with at least one reinforcing layer of martensite sheet steel which is interposed between the woven fabric layers.
While these prior art schemes are successful in defending a wall structure from impacting projectiles of many types, none are satisfactory (weight-wise) for protecting against orbiting particles with densities of about 2.7 g/cm.sup.3 and impact velocities up to 16 km/s.